Corynespora cassiicola causing leaf spots on Coleus barbatus

Coleus barbatus Benth (local name falso-boldo) is a tropical aromatic shrub of the Lamiaceae. It is widely used as a medicine against liver and digestion ailments in Brazil. Little attention has been given to plant diseases affecting medicinal plants in Brazil. Samples of C. barbatus from two separate locations at Viçosa (state of Minas Gerais, Brazil) showing seemingly undescribed leaf spots were collected in February and June of 2001.

Lesions were initially brown and punctiform becoming elliptic, subcircular to irregular and pale brown. They were well delimited with a dark brown rim (up to 5 mm in diameter) (Fig. 1), distributed on the lamina, sometimes coalescing and leading to extensive necrosis and yellowing. A dematiaceous fungus was consistently found sporulating on the centre of those lesions.

The fungus had conidiophores which were amphigenous, cylindrical, straight to slightly curved, unbranched, 101.0-227.5 x 6.5-9.5 µm, 4-12 septate, smooth and pale brown. Conidia were isolated or forming acropetal chains, fusiform to subcylindrical, 57.0-180.0 x 12.5-16.0 µm, with a rounded apex and truncate base, 5-12 septate, yellowish brown to subhyaline and smooth (Fig. 2). This corresponds well with features described by Ellis (1971) for Corynespora cassiicola (Berk. & Curt.) Wei. The specimens were deposited in the herbarium at the Universidade Federal de Viçosa as VIC 26503 and VIC 26504.

The fungus was isolated on PDA where it sporulated well. Six healthy C. barbatus plants were brush-inoculated with a Tween 20 suspension (0.05%) containing 4.2 x 10⁴ conidia of C. cassiicola per ml. Two additional plants were brushed only with Tween suspension. After inoculation the plants were left for 48 hours in a dew chamber at 25^°C. Necrotic areas appeared on leaves of inoculated plants in less than 24 hours. Parts of the tissue developed leaf spots with the same appearance as those seen in the field whereas other parts of inoculated leaves developed an extensive blight (Fig. 3). This suggests that a toxin was present in the suspension as observed in the studies carried out for C. cassiicola f. sp. lantanae (Pereira et al. 2002).

No record of this disease was found in the main Brazilian list of fungi on plants (Mendes et al. 1998) or in the world literature. Therefore this represents the first record of this disease. C. cassiicola 'is a common and cosmopolitan species, specially abundant in the tropics, with a wide range of host plants' (Ellis, 1971) and this represents another addition to its host range.